[X86][SSE] Vectorized v4i32 non-uniform shifts.

[X86][SSE] Vectorized v4i32 non-uniform shifts.

While the v4i32 shl operation is already vectorized using a cvttps2dq/pmulld pattern, the lshr/ashr opeations are still scalarized.

This patch adds vectorization support for non-uniform v4i32 shift operations - it splats constant shift amounts to allow them to use the immediate sse shift instructions, or extracts/zero-extends non-constant shift amounts. The individual results are then blended together.

Differential Revision: http://reviews.llvm.org/D11063

llvm-svn: 241989